The present invention relates to an electrostatic image-developing toner and a developer containing the same, used for developing electrostatic latent images formed in electrophotography, electrostatic recording, etc.
The developer used for electrocopiers, etc. is once deposited on an image carrier such as a photoreceptor on which an electrostatic image has been formed, in the developing step, then the deposited developer is transferred from the photoreceptor to a transfer sheet in the transfer step and the transferred developer is finally fixed on a copying paper in the fixing step. As the developer used for developing the electrostatic image formed on a latent image carrier, there are known two-component developer comprising a carrier and a toner, and one-component developer (magnetic toner) which requires no carrier.
For preparing a toner which is positively charged, a method is known in which a charge-imparting agent is added either internally or externally to a binder resin and a colorant. As the charge-imparting agent, there are known, for instance, Nigrosine dye, triaminophenylmethane compounds and quaternary ammonium salts.
Among the said charge-imparting agents, the quaternary ammonium salts, as compared with other agents such as Nigrosine dye, have advantages that they can be used for color toner as they are colorless and that they maintain good charging stability in continuous copying operations. But, on the other hand they have a drawback that the amount of the charge imparted is relatively small.
As the carrier used in combination with toner in development using a two-component developer, there are known iron powder, ferrite powder and the like. Ferrite powder is low in charge-imparting performance in comparison with iron powder.
Thus, in case of using a toner containing a quaternary ammonium salt, or in the case of a developer comprising the said quaternary ammonium salt-containing toner and a carrier composed of a ferrite powder, there has been the problem that no sufficient charge could be obtained, and especially in the case of using under a high temperature and high humidity conditions or in continuous copying operation, the charge was apt to decrease, causing to make background (BKG) level high or making it unable to obtain a stabilized copy density. Particularly at a time just after start of operation of a copying machine, as in the morning, it would be experienced that the copies be blotted in black due to increase of the BKG level or the brush marks be formed on the solid black of the copy.
As a result of strenuous studies for solving these technical problems, it has been found that by incorporating into a toner containing a quaternary ammonium salt, an acidic carbon black whose use as a component of a positively charged toner has rather been avoided since it is in itself an electron acceptive substance and has the nature to be charged negatively, the obtained developer shows an excellent charging characteristics and is capable of providing excellent image quality and durability. The present invention has been attained on the basis of this finding.